In internal combustion engines, the bearing assemblies typically each comprise a pair of half bearings retaining a crankshaft that is rotatable about an axis. Each half bearing is a hollow generally semi-cylindrical bearing shell, and typically at least one is a flange half bearing, in which the bearing shell is provided with a generally semi-annular thrust washer extending outwardly (radially) at each axial end. In other bearing assemblies it is also known to use an annular or circular thrust washer.
The bearing surfaces of bearing shells generally have a layered construction, in which a strong backing material is coated with one or more layers having preferred tribological properties to provide a bearing surface that faces a cooperating moving part, a crankshaft journal, in use. Known bearing shells have a substrate comprising a backing, which is coated with a lining layer, which is in turn coated with an overlay layer.
The strong backing material may be steel, having a thickness of about 1 mm or more.
A known lining layer may be a copper-based material (e.g. copper-tin bronze) or an aluminium-based material (e.g. aluminium or aluminium-tin alloy), which is adhered to the substrate (either directly to the backing or to an optional interlayer). The thickness of the lining layer is generally in the range from about 0.05 to 0.5 mm (e.g. 300 μm of copper-based alloy of 8% wt Sn, 1% wt Ni, and balance of Cu, apart from incidentally impurities).
The overlay layer may be 6 to 25 μm of a plastic polymer-based composite layer or a metal alloy layer (e.g. a tin-based alloy overlay).
For example, WO2010066396 describes a plastic polymer-based composite material for use as an overlay layer on a copper- or aluminium-based lining layer, which is in turn bonded onto a steel backing. The described overlay layer comprises a matrix of a polyimide/amide plastic polymer material, having distributed throughout the matrix: from 5 to less than 15% vol of a metal powder; from 1 to 15% vol of a fluoropolymer particulate, the balance being the polyimide/amide resin apart from incidental impurities (e.g. a layer of 12 μm thickness that comprises 12.5% vol Al, 5.7% vol PTFE particulate, 4.8% vol silane, <0.1% vol other components, and balance (approximately 77% vol) polyimide/amide).
Such plastic polymer-based overlay layers may be deposited by various different methods, including spraying, pad printing (an indirect offset printing process, e.g. in which a silicone pad transfers a layer of the plastic polymer composite material onto the sliding bearing substrate), screen printing, or by a transfer rolling process. Prior to deposition, the plastic polymer is in solution in a solvent, and the solid particulate is suspended in the solution.
After the deposition of the overlay layer has been completed, the entire polymer layer is thermally cured by heating to set the polymer-based layer, by inducing cross-linking of the polymer matrix. For example the overlay layer may be cured at 140 to 240° C. for a duration that may range from a few minutes to a few hours (e.g. 10 minutes to 2 hours). Different curing temperatures may be used when the polymer has been deposited on different metallic substrates. For example: for polymer deposited directly onto a steel backing without a lining layer, or where a copper-based lining layer is provided on the backing, the metallic substrate is able to withstand high temperature curing of the polymer; in contrast, where an aluminium-tin lining layer is used on the backing, lower temperature curing may be used, to avoid migration of the tin to crystal boundaries of the lining layer.
A particular challenge to the performance of bearing lining layers and/or overlay layers is provided by the fuel-saving configuration of vehicle engines to “stop-start” operation, in which the engine is stopped and requires restarting each time the vehicle stops, in contrast to conventional engine operation, in which the engine is kept running throughout a vehicle's journey. Engines configured for stop-start operation may restart their engines about one hundred times more frequently than conventionally configured engines, which are run continuously throughout each vehicle journey. Engine bearings are conventionally hydrodynamically lubricated, with little or no lubrication initially being provided to the bearings when the engine starts. Accordingly, stop-start operating of an engine can place increased demands upon the performance of the bearings.